The present invention relates to an apparatus for electrostatic spraying of a coating product such as, for example, paint, and concerns more particularly an improvement which prevents the creation of electric arcs that can represent a real danger when the coating product contains volatile and flammable ingredients.
An electrostatic spraying apparatus includes, in particular, a mobile sprayer furnished with a high voltage electrode and arranged to create an electric field between the high voltage electrode and the object to be coated, the latter being connected to electrical ground, and to disperse the coating product. In order to do this, the spraying apparatus comprises, in addition to the electrode, means for supplying the coating product, a medium, such as air, for entraining the coating product and electric energy. The sprayer is thus connected to a stationary supply and control unit by a bundle of electric cables and various conduits. The electric energy is almost always supplied in the form of a direct high voltage of the order of 50-150 kV, which is applied to the electrode. The resulting electric current, including that carried by the flow of charged coating product particles and ions created or transported through the atmosphere under the influence of the electric field, is of the order of 50-200 .mu.A. Several techniques have been proposed for applying the high voltage to the sprayer electrode.
The high voltage can be applied directly to the electrode from a high voltage generator located in the supply and control unit which is disposed at a distance from the sprayer and thus from the electrode. The generator encompasses components for converting alternating current energy supplied by the power supply mains into a high dc voltage. The supply bundle mentioned above then necessarily comprises a high voltage cable enclosed in a suitably designed high voltage insulating sheath.
It is also possible to supply a very low direct current voltage, of the order of several tens of volts, to the sprayer if the sprayer contains a cascade arrangement of a high frequency amplifier which is fed with this low dc voltage, a step-up transformer producing a medium level high frequency voltage of the order of several kV, and a conventional rectifier-voltage multiplier assembly, composed of capacitors and diodes, for transforming the medium level high frequency voltage into a high dc voltage, this high voltage being applied to the electrode or electrodes. In this case, the supply and control unit contains only a step-down transformer and a rectifier to convert the low ac voltage from the supply mains into a very low dc voltage. The previously mentioned supply bundle then includes conductors which do not have substantial insulation for transporting the dc current to the oscillator.
It has also been proposed to place only the transformer and the rectifier-voltage multiplier assembly in the sprayer. The oscillator is then disposed at a distance from the sprayer in the supply and control unit. In this case, the supply bundle carries an ac signal at very low voltage and at high frequency. The present invention, which will be described below, is particularly applicable to such an arrangement, even though the principle of the invention can be applied to other systems comprising a current supply for a high voltage electrode.
The electric arcs observed during operation of such installations can have various causes:
Regardless of the nature of the device utilized, it can occur that the distance between the high voltage electrode of the sprayer and the workpiece to be coated, which is at ground potential, becomes insufficient to withstand the high dc voltage; PA1 If the high voltage is developed at a distance from the sprayer, a fault in the high voltage cable insulation can also give rise to electric arcs; PA1 On the other hand, if the high voltage is developed in the sprayer, the bundle carries a relatively high current, of the order of several amperes, and a break in the conductor or a poor connection contact is capable of provoking an electric arc due to the energy stored in the circuit inductance.
Various systems have been proposed for counteracting the formation of electric arcs. These known systems are distinguished from one another on the basis of the type of electric arc which it is desired to eliminate and according to the quantity being monitored.
French Patent Application No. A-2158031 discloses monitoring the values of the electric current at the output of a high voltage transformer and cutting off the supply of the high voltage generator when the value of the current exceeds a threshold. U.S. Pat. No. 4,737,887 proposes monitoring variations with respect to time of the alternating current circulating in the conductor connecting the low voltage unit to the rectifier-voltage multiplier of a generator. One problem in monitoring systems of this type resides in the fact that in the event of a failure of the monitoring system, a potentially dangerous operating state, i.e. capable of leading to the creation of electric arcs, cannot be detected. The monitoring loop is not completely reliable.
Similarly, U.S. Pat. No. 4,402,030 proposes to measure variations of the current in the high voltage circuit of the generator but does not permit detecting poor operation of the control circuit. In particular, the detection of potentially dangerous situations is inhibited during starting of the generator.